Process of dewatering sewage sludge

ABSTRACT

Sludge from a municipal or industrial sewage treatment facility is mixed with a mineral acid or anhydride thereof to release the entrapped water in the sludge. The remaining mixture of sludge solids and diluted acid or anhydride is then mixed with a basic material, such as ammonia, the heat generated by reaction of the base and acid evaporates the water to form either a completely dry mixture of sludge solids and a salt, or a mixture having a predetermined moisture content which may be air dried. In some applications it is preferable to recycle a portion of the end product by adding it to the reaction vessel.

RELATED PRIOR APPLICATION

This is a continuation-in-part of U.S. application Ser. No. 526,134 ofthe same inventor, filed Nov. 22, 1974, and now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to sewage sludge disposal and, morespecifically, to novel methods of dewatering and forming useful productsfrom sludge produced by typical industrial or municipal sewage treatmentsystems.

Disposal of sewage sludges presents problems both of environmentalpollution and of energy consumption. Although transporting sludges bybarge several miles off coast and dumping it in the ocean is practisedin many coastal areas, the resulting pollution is recognized as aserious environmental problem which cannot continue indefinitely withoutdisastrous consequences. Likewise, land disposal by spreading wetsludges over unoccupied areas creates pathogenic and bacteriologicalrisks, as well as esthetic problems. In any case, transportation andhandling of the sludges, which may contain 90% or more of water, isexpensive and energy consuming. In spite of the economic and ecologicalobjections to water and land disposal of the sludges, they areconsidered preferable to drying or burning the sludge by heat fromburning fuels due to the enormous quantity of energy required toevaporate the entrapped water.

Recent methods of treating sludge to reduce the disposal problemsinclude acid treatment systems such as that disclosed in U.S. Pat. No.3,772,191, wherein the sludge is acidified with a mineral acid andallowed to separate by gravity into an upper layer of hydrophobicsubstances, an intermediate layer of supernatant liquid, and a lowerlayer of sludge substances of lower water content than the originalsludge. The upper and intermediate layers are drained off and theprecipitated sludge substances, still containing 80% water, are treatedwith calcium hydroxide to facilitate further dehydration throughpressure filtration in a filter press. The product mixture removed fromthe filter press has a water content of 30% to 50%, which is still50-65% water based on contained sludge in the total mixture.

Although the above described process constitutes a material improvementover conventional disposal methods through improved pressure filtrationrates of sludge solids, there are still a number of obvious problems.The surface layer, consisting of lipides, fatty acids, higherhydrocarbons and the like, must be disposed of, as must the intermediatelayer of diluted acid. Also, the final product, consisting of sludgesolids and lime, still may contain up to 50% water. Since the acidifiedsludge must be allowed to settle for several hours or days, the processis not suitable for continuous operation, with new materials constantlybeing added to mixing or reacting vessels.

It is a principal object of the present invention to provide a methodfor dewatering sewage sludge in a rapid, efficient, economical manner,essentially obviating all environmental and energy problems commonlyassociated with sludge disposal.

A further object is to provide a method of treating sewage sludge toobtain a dry end product having useful properties as a soil fertilizer,with no intermediate or by-products which create pollution or otherenvironmental problems in their disposal.

Other objects are to provide a sludge treatment process which may becarried out with the apparatus of conventional fertilizer granulationplants without substantial modification; to provide a method ofdewatering sewage sludge which may be practised in the manner of acontinuous chemical process; to provide a method of dewatering sewagesludge which utilizes materials other than the sludge which presentlyconstitute atmospheric pollutants; and, to provide a process fortreatment of sewage sludge to yield a dry mixture of sludge solids andfertilizer salts wherein heat provided by a chemical reaction serves tosterilize the product.

Other objects will in part be obvious and will in part appearhereinafter.

SUMMARY OF THE INVENTION

In accordance with the foregoing objects, the invention may be practisedin the setting of a typical fertilizer granulation plant. The sludge andmineral acid, or an anhydride thereof, are added simultaneously to anagitated preneutralizer initially containing a quantity of the acid oranhydride suitably diluted.

In a preferred embodiment oxides of sulphur are mixed with the sludge,thus providing an essential constituent of the process while at the sametime disposing of compounds which could otherwise become atmosphericpollutants.

The acid or anhydride serves to free the entrapped water from the sludgemolecules and an essentially homogenous mixture of the sludge solids inthe water-diluted acid is formed. The mixture from the pre-neutralizeris added, together with a quantity of a basic chemical, preferablyammonia, to a conventional rotary granulator. The heat from the reactionof the acid (or anhydride) and base is sufficient to evaporate all orsome desired portion of the water initially contained by the sludge.Although it is frequently desirable that the product removed from thegranulator have some moisture content, conventional dryers and/orcoolers may be utilized to produce a completely dry end product. In mostapplications of the invention, efficiency and performance may beenhanced by recycling a portion of the end product through thegranulator.

Other refinements of the invention include using hot air from acondenser for drying the product removed from the granulator, thecondenser being used in conjunction with the pre-neutralizer inconventional fashion. Also, the hot, humid air emerging from the dryermay be utilized in the granulator, and hydrogen sulfide derived from themixture in the pre-neutralizer may be mixed with the ammonia prior toadding the latter to the granulator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process diagram of a typical application of the invention;

FIGS. 2 and 3 are a set of curves illustrating certain relationships inthe practise of the invention when employing 100% sulfuric acid andammonia; and

FIGS. 4 and 5 are a set of curves illustrating the same relationshipswhen employing a sulfur trioxide and ammonia.

DETAILED DESCRIPTION

Referring now to the drawings, the reference numeral 10 denotes a mixingvessel such as a conventional agitated pre-neutralizer of a typicalfertilizer plant. Sewage sludge is added at a controlled rate,determined in accordance with considerations discussed later herein, tomixing vessel 10 through inlet line 12. A mineral acid or anhydridethereof is added through inlet line 14 for mixing with the sludge. Inprocesses utilizing an anhydride, the water contained in the sludge willimmediately combine with the anhydride to form the acid, whereby amixture of sludge and diluted acid results in either case. Aconventional heat exchanger 16 receives cold air or atmospheric air atambient temperature through line 18, which condenses the volatilesubstances which are boiled off by the heat generated by mixing thesludge with the acid or anhydride. The vapor from vessel 10 is suppliedthrough line 20 and condensate is returned to vessel 10 through line 22.Heated air leaves heat exchanger 16 through line 24.

The mixture of sludge and acid/anhydride is fed from vessel 10 throughline 26 to a reaction vessel 28 preferably in the form of a typicalfertilizer plant rotary granulator. A controlled quantity of a basicchemical, such as ammonia, is added to reaction vessel 28 through line30 simultaneously with the mixture of sludge and acid. Line 27 isprovided to allow selective addition of any non-condensible substancesfrom heat exchanger 16 to vessel 28, as desired. Also, selectedquantities of the basic chemical may be added to the materials in vessel10 through line 29, in cases where it may be desirable to pre-neutralizethe acid to some extent before the materials are added to vessel 28. Itwill be understood that suitable valves are provided at appropriatelocations in any or all of the inlet or outlet lines. The acidic andbasic materials react in vessel 28 to form a salt and water, the heat orreaction serving to evaporate all or some of the water initiallycontained in the sludge as well as that produced in the reaction. Theheat of the reaction also serves to effectively sterilize the materialsin vessel 28, thus eliminating or minimizing any problems frompathogenic substances which may be present in the sludge. In accordancewith the usual practise in commercial fertilizer production, themoisture content may be controlled to provide a desired degree ofplasticity of the product as it emerges from reaction vessel 28. Thewater vapor is exhausted through vent 31.

The product passes from vessel 28 to a conventional dryer 32, asindicated by line 34. Relatively dry hot air is supplied to dryer 32,which may conveniently be connected to line 24 to utilize the air heatedin exchanger 16. Moist hot air is exhausted from dryer 32 through line35 and may be fully or partially returned to vessel 28, if desired.After being dried completely, or to some degree of desired low moisturecontent, the product may be cooled in cooler 36 to which air is suppliedthrough line 38. The warmer air exhausted through line 40 may be fullyor partially returned to dryer 32, or vented to the atmosphere.

The product emerging from cooler 36 will be a dry mixture of sludgesolids and fertilizer salts. The product may then be passed through aconventional screening operation, as diagrammatically indicated at 42for separating the granules into different size groups. A portion of thefinal product is taken from the screening operation for return, via line44, to reaction vessel 28, where it is mixed with the other substancesentering through lines 26 and 30, as previously described. Thisrecycling of the final product assists in providing a desiredtemperature and moisture content within vessel 28 to produce thenecessary consistency for granulation of the product within vessel 28.

The process is preferably carried out in a continuous manner. That is,the sludge and other raw materials are continuously added at acontrolled rate and flow through the consecutive stages of the processin a continuous manner. The rate at which the materials are processedwill depend upon such factors as the initial water content of thesludge, the desired percentage of sludge solids in the final product,the particular chemicals used and the desired amount of moisture to beremoved by heat generated by the reacting materials. The curves shown inFIGS. 2-5 provide an indication of how the various process parametersmay be relatively controlled.

The curves shown in FIGS. 2 and 3 use the same horizontal scale, i.e.,the initial water content of the sludge being processed in terms of theweight of the water as a percentage of the total weight of the sludge.The curves of FIGS. 2 and 3 apply to a process wherein the chemicalmixed with the sludge in vessel 10 is sulphuric acid of 100%concentration prior to being mixed with the sludge. The four curves inFIG. 2 relate the initial water content of the sludge to the resultantconcentration of the acid after dilution by the water in the sludge fora desired level of sludge solids in the final product. The verticalscale on the left side corresponds to the concentration of the dilutedacid for the various levels of sludge solids as a percentage by weightof the final, dry product indicated by the four curves. For example, ifthe sludge is initially 80% water, point 50 on the lower curve indicatesthat if the end product is to be 33% sludge solids, the concentration ofthe acid after mixing with the sludge will be 26 %. For the same initialwater content, if the end product is to include 11% sludge solids, theacid concentration will be 60%, as shown by point 52 on the upper curve.The vertical scale on the right side is provided to show the boilingpoint of the acid at the concentration indicated at the same level onthe left vertical scale. That is, the boiling point of sulphuric acid at60% concentration is 284° F.

The four curves in FIG. 3 which lower, or decrease, from left to rightcorrespond to the vertical scale on the right. These curves relate theinitial water content of the sludge for a desired level of sludge solidsin the final product to the recycle ratio, i.e., the percentage of theend product which is to be fed back to reaction vessel 28 in order toprovide the desired process parameters such as temperature, moisturecontent, and plasticity. For example, for sludge having an initial watercontent of 80%, if the final product is to consist of 33% sludge solids,point 54 on the 33% parameter (upper curve) indicates that about 9 lbs.of the dried sludge/salt mixture per lb. of final product must berecycled to vessel 28. If the initial water content is 90%, then 20 lbs.of dried sludge/salt mixture per lb. of final product is recycled asindicated by point 56 on the same curve.

The four curves which rise or increase toward the right in FIG. 3 relatethe initial water content of the sludge to the percentage of the totalheat required for complete drying of the product formed in reactionvessel 28 which is available from the heat of the reaction itself forvarious levels of sludge solids in the final product. For example, point58 on the curve corresponding to a sludge solids content of 17%indicates that approximately 100% of the required heat is provided bythe reaction for sludge which is initially 80% water by weight. If thesludge solids content of the product is higher, a lower percentage ofthe required heat is available from the reaction in vessel 28.

The curves of FIGS. 2 and 3 represent the various describedrelationships between process parameters when the chemical mixed withthe sludge in mixing vessel 10 is sulfuric acid at an initialconcentration of 100%. The curves of FIGS. 4 & 5 illustrate the samerelationships when sulfur trioxide is the chemical mixed with the sludgein vessel 10. It should also be pointed out that the recycle ratiosrepresented on the right side vertical scales of both FIGS. 3 & 5 arecalculated on the assumption that the materials combined in reactionvessel are 35% liquid phase and the temperature within vessel 28 ismaintained at 275° F. The recycle ratio will vary if these parametersare changed. It is preferred that the temperature within vessel 28 bemaintained at 250° F. or higher in order to provide proper granulation.

In order to provide a fuller understanding of the invention, a pair ofspecific examples of actual practise of the method will now be given. Inboth examples the apparatus utilized is that of a typical fertilizergranulation plant and, since the process is to be carried on in acontinuous manner, the pre-neutralizer of the plant is initially chargedwith a quantity of chemical such as the acid or anhydride to be mixedwith the sludge at about the concentration it would have after dilutionby the water in the sludge.

EXAMPLE I

A conventional agitated preneutralizer of a typical fertilizergranulation plant initially contains refluxing sulphuric acid of about30-35% concentration at 225°-230° F. To this is added sulphur trioxidegas at a rate of 22.6 tons per hour and sewage sludge having a watercontent of 85% by weight at a rate of 83.3 tons per hour. The sulphurtrioxide combines with some of the water initially contained in thesludge to form sulphuric acid, diluted by the remainder of the sludgewater. The mixture from the preneutralizer is added to the fertilizerplant granulator simultaneously with ammonia at a rate of 9.7 tons perhour and recycled, dry material from the end product of the process at arate sufficient to maintain the temperature of the material in thegranulator at 250°-275° F., which will typically constitute about 85 to90% of the final product. The hot, moist, granular material is thendried, using hot air generated in the process, and passed through acooler. The chemical reaction within the granulator provides about 75%of the energy required to evaporate the remaining water at a rate of 65tons per hour. The final product, obtained at a rate of about 50 tonsper hour, is a granulated mixture of 75% ammonium sulfate and 25% sludgesolids.

EXAMPLE II

Sewage sludge of 80% water content is added at 42.4 tons per hour and100% concentrated sulphuric acid is added at 30.7 tons per hour to apreneutralizer containing sulphuric acid at about 50% concentration,refluxing at about 250° F. The acid/sludge mixture is added to thegranulator together with 10.8 tons per hour of ammonia, and recycled drymaterial. Final product is obtained at a rate of 50 tons per hour, ofwhich 83-86% is recycled through the granulator. The final product is83% ammonium sulfate and 17% sludge solids. The chemical reactionprovides all of the heat required to evaporate the 34 tons per hour ofwater generated in the process.

From the foregoing detailed description and examples, it is apparentthat the process provides an effective and economical dewatering anddisposal system for typical municipal and industrial sewage sludges. Theprocess is carried out through conventional apparatus currently incommercial use, requiring little or no adaptations. The water isevaporated largely or fully by heat generated naturally in the course ofthe process, thus requiring little or no supplemental energyconsumption. The process produces no harmful pollutants, or the like,the only significant exhaust gases being water vapor. In fact, besidesdisposing of and utilizing sewage sludges, the process may utilize otherpotentially harmful compounds such as sulfur trioxide. The final productconsists of fertilizer salts and sludge solids having useful soilnutrient properties.

What is claimed is:
 1. A process of dewatering non-compressible sewagesludge initially containing a known percentage by weight of water but nogaseous air phase to form an end product containing a predeterminedpercentage by weight of sludge solids, said process consistingessentially of:a. physically mixing a quantity of sewage sludge having awater content sufficient to render it non-compressible with a quantityof mineral acid or anhydride thereof sufficient to extract substantiallyall of the water initially contained in the sludge to form a mixture ofsludge solids and water diluted acid; and b. placing in a reactionvessel said mixture and a base of a type and quantity sufficient toreact with the acid or anhydride contained in said mixture to generate areactive heat sufficient to remove by evaporation all of the waterremaining in said mixture required to allow formation, withoutfiltration, of a granular product comprising sludge solids and a salt.2. The invention according to claim 1 wherein said acid or anhydride isselected from the group consisting of phosphoric acid, phosphorous acid,sulfuric acid, sulfurous acid, sulfur dioxide, and sulfur trioxide. 3.The invention according to claim 2 wherein said base is ammonia.
 4. Theinvention according to claim 1 wherein said predetermined quantity ofwater evaporated constitutes substantially all of the water contained insaid mixture.
 5. The invention according to claim 1 wherein saidpredetermined quantity of water evaporated constitutes substantiallyless than all of the water contained in said mixture, and including thefurther step of air drying the product after reaction of said base withthe acid or anhydride contained in said mixture.
 6. The inventionaccording to claim 1 wherein the temperature of the reacting massgenerated by said reactive heat is at least 250° F.
 7. The inventionaccording to claim 1 and further including recycling a predeterminedportion of said product by adding said portion to said reaction vesselsimultaneously with said mixture and base.
 8. The invention according toclaim 7 wherein the relative proportions of said recycled portion ofsaid product, said mixture and said base are balanced to maintain atemperature and moisture content of the mass within said reaction vesselyielding a desired consistency of said mass.
 9. The invention accordingto claim 5 and including the further step of condensing in a heatexchanger at least a portion of any volatile substances which are boiledoff by the heat generated by mixing the sludge and acid or anhydride.10. The invention according to claim 9 and further including providingat least a portion of the heated air from said heat exchanger to performsaid air drying step.